How to prepare model for 3D print?
Have you tried the blender 3d printing add on?. If that hasn't worked, I'd suggest exporting a selection and attempting a different workflow. That article's goal is to create a mold, but the software it uses (Meshlab, Meshmixer) can help you accomplish some tasks. Some very complex models I have found hard to do everything automatically. Further, if you are working with slicing, I would recommend chitubox. It automatically hollows models and creates supports.